1. Field of the Invention
The present invention generally relates to methods for sizing the depth of cracks in thin walled tubes or plates and more particularly to improved methods for measuring small cracks in both thin wall tubing and thin wall plates.
2. Description of the Prior Art
The presently known sizing methods for detecting cracks in thin wall tubes are based on transverse (shear-wave) ultrasonic waveform analysis.
The analysis of shear wave data presently uses three basic methods to estimate the depth of such a crack. The three methods are identified as tip diffraction method, the multiple skip method and the target motion time of flight (TOF) method. They are all applicable to pulse echo ultrasonic testing methods.
Cracks in thin walled tubing typically have openings or gaps that are less than 0.001 in. deep and are propagated normal to the crack initiation surface. Ultrasonic testing methods use shear wave techniques to detect and depth size these cracks. These techniques transmit an ultrasonic angle beam pulse (shear wave) which is propagated within the wall in a path that resembles a xe2x80x9cVxe2x80x9d as best seen in FIG. 1. In this figure, the time of flight (TOF) equals the time difference between the incident inner surface reflection and the corner reflector formed by the outer diameter surface and the crack intersection.
The use of a 45 degree shear wave angle shown in FIG. 1 has been shown to be the optimum angle for the detection of planar flaws in thin wall tubes. Refracted angles less than 45 degrees reduce the reflected energy from planar flaws that propagate normal to the tube wall. Refracted angles greater than 45 degrees provide a longer transmission and attenuation path. Refracted angles less than 45 degrees have reduced reflected signal amplitude and have a reduced ability to separate closely spaced flaws. For example, smaller angles (normal or zero degrees beam) are blind to tight crack like flaws.
The most accurate known depth sizing method is the tip diffraction method. It is based on the detection of a reflection signal from the crack""s tip. This method is best understood by referring to FIG. 2 for the following discussion.
The detection of a crack tip signal is indicated therein by a return signal that has a waveform reflection from the crack""s tip in addition to a waveform reflection from the crack""s corner reflector. This crack tip signal is rarely observed in real cracks for two reasons. First, the reflection from the tip requires a minimum crack width, or gap, of one tenth of a wavelength. The second reason is that a tip reflection signal cannot be distinguished from the corner reflector signal until the crack depth of penetration exceeds one wavelength. The time of flight for these two reflection signals overlap until this depth of penetration is achieved. For a 10 Hz transducer, the depth of penetration equivalent to one wavelength is 0.012 inches, and the minimum gap is equal to 0.0012 inches. For depths of penetration greater than one wavelength, the tip reflection signal should be distinguishable from the corner reflector signal.
Unless both of the above conditions are met, it is unlikely that a crack tip signal can be detected and used for sizing small cracks.
The multiple skip method displayed in FIG. 3 recognizes the presence of the three skip reflections as an indication of a crack""s depth of penetration. As the depth of penetration increases, the amplitude of the inner diameter (full) response increases relative to the outer diameter (half and one and one half) skip amplitudes. When a crack produces significant reflection signals at the half skip, full skip, and a one and one half skip positions, the depth is considered to be near through wall. This association of multiple skips and deep depth of penetration has destructive evaluation (DE) support. Based upon the theory that any reflector of depth sufficient to trap a wavelength will produce a significantly large amplitude return, significant return signals should be received when the remaining wall is less than 0.012 inch for a typical 10 MHz transducer.
The Target Motion Time of Flight Method uses the half skip""s target motion time of flight information to determine the crack""s depth of penetration. In the absence of reflections at the expected full skip, the analyst estimates depth using the earliest or latest detected reflections along the target motion for the outer diameter, half skip signal. This method assumes that the reflections are forty-five degree shear wave returns from the crack face. It also assumes that the earliest and latest detection reflections are coincident with the crack""s tip. The crack depth is the difference between the measured depth of the corner reflection and either of the earliest or latest detection. Target motion TOF is the most accurate method for determining the location of a corner trap. However, the method may not accurately locate the crack tip, resulting in the failure to accurately depth size the crack.
Given an acceptable 45 degree shear wave calibration, where the depth associated with each skip is a multiple of the wall thickness, the estimated crack depths should be equal to the transducer displacement along the inspection axis.
The above discussed methods are more fully described in the following U.S. patents and the reader is referred thereto for a more detailed discussion. In summary, U.S. Pat. No. 5,125,272 teaches the use of tip diffraction and longitudinal waves to determine the depth of a crack in a tube surface. U.S. Pat. No. 4,658,649 teaches an inspection system for surface cracks utilizing both shear waves and longitudinal waves. This reference also teaches the conversion of shear waves to longitudinal waves and the use of creeping longitudinal waves. U.S. Pat. No. 5,467,321 teaches the use of ultrasonic transducers with a mode conversion.
The discussed known methods have defects which do not allow for an accurate measurement of small cracks. In summary:
1. For thin wall tubing, crack gap is typically less than {fraction (1/10)} wave length so the known tip method is rarely available for such sizing.
2. The multiple skip method provides only an estimate of the crack depth. The multiple skip indicates that the crack is xe2x80x9cdeepxe2x80x9d or mathematically the crack depth is within one wave length of being through wall. No actual depth determination can be made using this method.
3. The target motion TOF method makes assumptions concerning the nature of the reflected signal that may be incorrect. Also, the earliest and latest detection may not coincide with the tip of the crack.
Thus an improved method of thin wall tube crack sizing was needed.
The present invention solves the problems associated with the discussed prior art sizing methods and other by providing a crack sizing method which uses a combination of depth crack sizing methods to improve crack sizing accuracy for thin walled tubing and tight (crack surface opening less than 0.001 inches) tube cracks of any depth. The tube wall can consist of a single material or multiple metallic electrodeposited materials with different magnetic properties such as Electrosleeve(trademark).
The present method uses a combination of known depth sizing methods including the shear wave time of flight method and two new depth sizing methods. The two new methods are designated as the mode converted signal (MCS) method and the full skip normalization (FSN) method which when used in combination will measure thin wall tube cracks accurately as well as crack openings less than 0.001 in. for any thickness of tube even ones made up of differing material layers (electrodeposited or otherwise intimately bonded).
In view of the foregoing it will be seen that one aspect of the present invention is to provide an improved accuracy method for thin wall tubing sizing.
Another aspect is to provide an accurate method of measuring crack surface openings less than 0.001 in. for any thickness tube or plate.
Yet another aspect is to provide an improved accuracy crack sizing method for multiple material layered plates or tubes (electrodeposited or otherwise intimately bonded).
These and other aspects of the present invention will be more fully understood after a perusal of the following preferred description of the invention when considered in conjunction with the accompanying drawings.